1. Field of the Invention
The present invention relates to an optical lens assembly for image taking, and more particularly to an optical lens assembly composed of four lenses and applied to an electronic product, and the optical lens assembly has the features of short total length and low cost.
2. Description of the Related Art
As science and technology advance, the development of electronic products such as digital still cameras, web cameras, and mobile phone cameras comes with an optical lens assembly with a smaller size and a lower cost to meet user requirements, while providing an optical lens assembly with good aberration correction capability, high resolution, and high image quality.
In general, a conventional optical lens assembly for image taking for a mini electronic product has different designs including the two-lens, three-lens, four-lens, and five-or-more lens designs. If the image quality is taken in consideration, the four-lens and five-lens optical lens assembly provides better aberration correction and modulation transfer function (MTF), and the four-lens optical lens assembly uses less lenses and incurs a lower manufacturing cost than the five-lens optical lens assembly, and thus the four-lens assembly can be used in electronic products that require a high pixel quality.
In the design of various mini four-lens optical lens assemblies for image taking, the prior art adopts a combination of different positive or negative refractive powers, wherein the design having a combination of the first lens element with positive refractive power, the second lens element with positive refractive power, the third lens element with negative refractive power and the fourth lens element with positive refractive power as disclosed in U.S. Pat. Nos. US2007014033, US2008/0024882, U.S. Pat. No. 7,215,492 and U.S. Pat. No. 7,321,474, European Pat. No. EP1821129, Japan Pat. Nos. JP2007-225833, JP2008-020893, JP 2007-286153 and JP 2007-193195, R.O.C. Pat. No. TWM314860 and P.R.C. Pat. No. CN1815287 tends to give a good aberration correction. If the fourth lens element adopts a simple lens shape as disclosed in U.S. Pat. Nos. US2007/0058256, US2007/0070234, US 2007/0242370 and US2008/0043346, and Japan Pat. Nos.: JP2005-091666, JP2005-025174, JP 2004-233654 and JP2007-219520, the yield rate can be improved.
In compact digital cameras, web cameras or mobile phones, the optical lens assembly requires compact design, short focal length and good aberration adjustment. A design having a crescent or bi-concave first lens element and the second lens element with positive refractive power or other combinations as disclosed in U.S. patents, European patents, Japanese patents with Publication Nos. JP2004-061938, JP2007-157031 and JP2004-361934 and R.O.C. patents with Publication No. TW201024789 can meet the miniaturization requirement. However, the optical lens assemblies disclosed in these patents still required a reduction of the total length of the lens. These prior technologies adopt a direct way of shortening the rear focal length. Although the total length of the lens is effectively reduced, the drawback is that the aberration correction is difficult to improve or the image distortion is hard to lower down. On the other hand, the reduction of the total length of the lens can be accomplished by adopting a concave surface (or a convex surface) at the object-side surface of the third lens element or fourth lens element near the optical axis and a convex surface (or a concave surface) while approaching the rim of the lens, such that the curvature has a very large change, but such lens is very difficult to manufacture. Therefore, the present invention provides a more practical design to shorten the optical lens assembly while using a combination of refractive powers, convex surfaces and concave surfaces of the four lenses to reduce the total length of the optical lens assembly, and additionally to improve the image quality and use a simple shape of the lens to lower the manufacturing cost and applying the lens to electronic products.